1. Field of the Invention
The present invention relates to a magnetron, and more specifically to an improvement in a core type magnetron.
2. Description of the Prior Art
In order for a magnetron to oscillate at the rated frequency and output, the prescribed high voltage and the prescribed magnetic field must be applied. Application of the prescribed high voltage can be readily performed by controlling an electric circuit connected to a constant voltage supply. However, application of the prescribed magnetic field has been recently effected by means of a permanent magnet, which cannot provide a normally constant magnetic field, since it is influenced by the ambient temperature. More specifically, a cast magnetic material such as alnico-5 or a ferrite magnetic material has been used as a permanent magnet in prior art magnetrons. The reversible temperature variation coefficient of alnico-5 magnet is -0.021%/.degree.C. and that of ferrite magnet is 0.18%/.degree.C. As is well known, the reversible temperature coefficient is temperature dependent in the case of residual magnetic flux density (Br) and varies irrespective of the operation point of the magnet. In other words, such magnets are demagnetized with an increase of temperature.
In a magnetron, the anode cylinder thereof is elevated to 350.degree. - 400.degree. C. due to the heat caused by the anode loss, a cathode heater and the like during the oscillation thereof. Therefore, in case of the employment of a ferrite magnet the magnetic field is considerably decreased at that temperature elevation, with the result that the oscillation frequency fluctuates and the output undesirably varies. Therefore, in designing conventional magnetrons, consideration has been given to preventing the temperature of the magnet from being increased, to thereby prevent the magnet from being demagnetized by an increase in temperature. Thus, conventional magnetrons were of a shell type, wherein a magnet is provided outside and spaced from the anode cylinder. However, shell type magnetrons have the disadvantage of being large in size, which necessarily causes the magnetic path, for providing a magnetic field to the operation space within the anode resonator, to become long and increase the possibility of flux leakage.
Core type magnetrons comprising a magnet housed inside the anode cylinder have a much shorter magnetic path and can be made much smaller in size. Nevertheless, the core type magnetrons have heretofore been impractical to use because of the large amount of demagnetization which occurs due to an increase in the temperature of the magnet.